Anti-rewet membrane for an extended press nip system

ABSTRACT

A press mechanism and method for removing liquid from a traveling fibrous web of the type such as formed in a paper making machine and received from the fourdrinier section with the press providing an extended nip formed between a pair of traveling belts or between a belt and a roll with pressure applied to the web through the belts by hydraulic chambers or by the tension of a belt against the roll. A felt receives the liquid from the web, and a thin porous sheet of a predetermined porosity is placed between the web and felt to inhibit the flow of water between. The compressing time exceeds the release time so that a greater amount of water will flow from the web to the felt than will return to the web.

United States Patent 1191 1111 40, Bus ker et a1 1 Oct. 8, 1974 [54] ANTI-REWET MEMBRANE FOR AN 3,293,121 12/1966 Martin 162/358 EXTENDED PRESS NIP SYSTEM 3,748,225 7/1973 Busker et a1 162/358 [581' Field Inventors:

Assignee:

Filed:

Appl.

Leroy H. Busker; Robert A. Daane, both of Rockton, lll.

Beloit Corporation, Beloit, Wis.

Aug. 7, 1972 US. Cl 162/205, 100/120, 100/151,

162/358, 162/DIG. 1

Int. c1. D21f 3/02 of Search... 162/358, 205, DIG. 1; 139/383 A; 34/243 F; 100/151, 120

References Cited UNITED STATES PATENTS Wicker et a1.

Primary ExaminerS. Leon Bashore Assistant ExaminerRichard V. Fisher Attorney, Agent, or Firm-Hill, Gross, Simpson, Van Santen, Steadman, Chiara& Simpson [57] ABSTRACT A press mechanism and method for removing liquid from a traveling fibrous web of the type such as formed in a paper making machine and received from the fourdrinier section with the press providing an extended nip formed between a pair of traveling belts or between a belt and a roll with pressure applied to the web through the belts by hydraulic chambers or bythe tension of a belt against the roll. A felt receives the liquid from the web, and a thin porous sheet of a predetermined porosity is placed between the web and felt to inhibit the flow of water between. The compressing time exceeds the release time so that a greater amount of water will flow from the web to the felt than will return to the web.

14 Claims, 6 Drawing Figures ANTI-REWET MEMBRANE FOR AN EXTENDED PRES S NIP SYSTEM BACKGROUND OF THE INVENTION The invention relates to improvements in press mechanisms such as used in paper making machines wherein a pressure is applied to a traveling web to squeeze liquid therefrom and more particularly to such a structure wherein'rewetting, i.e., the return of water from the felt to the web is appreciably reduced.

In a conventional two roll wet press, the paper web is passed through the nip of the press with a felt and water is pressed from the web into the felt during the ingoing or converging region of the press rolls. During the diverging or outgoing region, some of the expressed water transfers back from the felt to the paper web. This transfer of waterfrom the felt to the paper in the expanding nip region is termed rewetting or rewet.

The driving force moving the water from the felt to the paper in rewetting is generally believed to be expanding voids which develop a vacuum in the expanding paper along with a capillary holding force favoring transfer to the paper.

In the two roll conventional wet press, the time of compression is nearly equal to the time of expansion inasmuch as the ingoing nip is equal in width to the outgoing nip. The present invention employs the principles of an extended nip presswhich offers the possibility of accomplishing a difference in the time period of compression relative to the time period for expansion. The time period of compression will be longer than the interval for expansion by a factor of or'more.

It is believed that rewetting is a function of time. That is, it is a flow phenomena and advantage can be taken of the difference of time of compression and the time of expansion to control the amount of rewet that occurs. This is accomplished by controlling or deterring the speed of travel of the liquid between the web and the felt so that by providing an adequate compression time even with a retarded flow of water from the web to the felt, adequate pressing action of the press and drying of the web will occur, yet the flow of water will be impeded sufficientlyso that rewetting is substantially and appreciably reduced.

It is accordingly an object of the present invention to provide a press structure for pressing water out of a traveling web which is capable of providing a dryer web by reducing the effects of rewetting.

By utilizing the principles and advantages of an extended nip press, improved drying can be obtained in the press, and this improved drying effect can be wholly retained and improved by employing the principles of the present invention. In the copending application of Busker and Francik, Serr No. 90,921, now US. Pat. No. 3,784,225, the principles and advantages of pressing a web for an extended period of time as discussed.

modifications can be made in this type of overall machine, as to the forming section, the press section. the dryer section, and the structure of the instant disclosure may be employed in pressing webs of various synthetic fibers. I e

The present invention relates to improvements for the press sections ofa paper making machine. In such a machine the web usually arrives at the press section with about 80 percent wet basis moisture (ratio of water to fiber plus water) and leaves the press section with approximately 60 percent moisture, with the remaining moisture having to be removed by thermal evaporation in the dryer section as the web passes over a series of heated dryer drums. Because of various inherent limitations in the operation of roll couples forming press nips for the press section in a conventional paper making machine, only a given amount of water can be removed in each nip and, therefore, in a conventional paper making machine, a series of three press nips are usually employed. It has been found impractical to attempt to remove a significant amount of additional water by increasing the number of press nips, although the further removal of Water by pressing can greatly reduce the expense and size of the dryer section. It is estimated that if the water removed in the press section can be increased to decrease moisture from 60 percent to percent, the drying load on the dryer section can be reduced by 1/3. This is significant in a typical 3,000 feet per minute newsprint machine which employs on the order of -70 dryer drums. This significance can be appreciated in considering that the dryer drums are each expensive to construct and to operate and require the provision of steam fittings and a supply of stream for each drum. The relative importance of the removal of water in the press section is further highlighted by the fact that one of the most important economic considerations in justifying a satisfactory return on investment in the operation of a paper making machine is to obtain the highest speed possible consistent with good paper formation and better pressing will shorten the necessary time in the dryer section and permit higher speeds.

It is accordingly an object of the present invention to provide an improvement in the press section of a paper machine which makes it possible to remove an increased amount of water in this press section and makes it possible to provide a press section having only a single pressing nip of a unique elongated orextended nature which does not have the performance limitations of conventional roll couple presses and which requires far less space in terms of requirements as to the overall length of the press section. By increasing the amount of water removed from the web in the press section, increased speeds are possible with existing equipment, i.e., a given length of dryer section can operate at higher speeds since it is required to remove less water. Also, new equipment can be constructed requiring less machine length and expense.

The present invention employs a principle which may be referred to as the extended nip concept wherein the time the web is subjected to a pressing action is greatly extended, i.e., a single pressing is provided having a residence time which exceeds thatof the time of the web I ample, the effects of the moisture in the felt which travels with theweb are small. It has been found further that the length of time that the web is in thenip, in other words the residence in the nip, can have a significant effect in overcoming the difficulties created by the flow properties of the water within the sheet. Ithas also been found that merely by increasing the residence time of the web in the nip, the water content of the sheet coming out of the press can be decreased so that the web will have 54 per cent moisture rather than 60 per cent moisture-with other variables remaining constant. As is evident, the residence time of a web in a conventional roll couple press nip is limited and can only be increased by decreasing the speed of travel of the web, or can be increased slightly by increasing the diameter of the press rolls, but these factors are indeed limiting. It has been found, for example, that by applyv inga 1,300 pound per square inch pressure on a web for minutes, a moisture level of less than 30 per cent can be attained. Yet, under the dynamic short term mechanical pressing of a paper machine press section using roll couples, even with a plurality of nips, a great deal of effort is required to maintain moisturelevels below 60 per cent.

It has been found that significant losses in dryness occur at higher speedsand that a loss in dryness of over 5 per cent is experienced in going from 300 feet per minute to 1,000 feet per minute with typical press loadings in a suction press. It has been found that a hydraulic pressure or wedge effect develops during the passage of the wet mat through the mat press nip. The hydraulic pressure that develops subtracts from the applied load and reduces the mechanical compacting pressure. The result is a loss in dryness. As the machine speed increases, the compacting rates are higher resulting in higher hydraulic pressures within the paper mat.

These hydraulic pressures react against the pressure of the rolls and prevent the moisture from being squeezed from the web. The exact value of hydraulic pressure is difficult to determine either by direct measure or analysis because ofthe space and speeds involved. It is believed, however, that hydraulic pressure predominately determines press performance on machines operating at high speeds. Accordingly, the instant invention relates to avoiding disadvantages encountered with high speed press nips of the conventional type used in most commercial applications today, and provides a substantial increase in residence time within a press nip to allow time for flow to occur within the mat and for the hydraulic pressure to'dissipate. The principles of extended nip or extended time pressing are further reviewed in the aforementioned copending application. It is a further object of the present invention to provide a press of the type above described which substantially avoids rewetting and retains the achievements of an extended nip press by employing a thin sheet of a predetermined porosity between the web and felt to re- 4 tard the rate of flow of water from the felt back to the web.

Other objects, advantages and features will become more apparent with the disclosureof the principles of the invention and it will become apparent'that equivalent structures and methods may be employed in connection with the description of the preferred embodiment teaching of the invention in'the specification, claims and drawings in which:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat schematic showing of a structure embodying the principles of the present invention;

FIG; 2 is a graph illustrating the relationship of compression time to expansion time in an extended nip press in accordance with the invention;

FIGS. 3 through 5 show schematically three additional separate embodiments of the invention; and

FIG. 6 is a schematic side elevational view of another form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a liquid containing web such as paper coming from a fourdrinier wire is shown at W passing through the nip of an extended nip press. The web passes through the nip with a porous felt F which receives water pressed from the web. The felt is formed in the loop and normally will have felt dewatering means which reduces'moisture although its moisture content has been found to not be critical.

In accordance with the principles of the invention, thin porous sheet S is passed between the web and'the felt to retard or control thetransfer of water between the felt and the paper. The. sheet is in the form of a belt and is provided with suitable guide rolls. This sheet may be of various materials such as porous stainless steel or plastic. While the belt retards the flowof water, since the compression time is substantially longer than the release or expansion time, adequate water flows through the sheet to the felt for dewatering, but the retardation of the flow of water will greatly reduce wetting that occurs during theexpansion time.

In the press nip which extends for the width of the bearings 16 and 17, hydraulic pressure is transmitted to the web through the belts l0 and 11. Belt 10 passes over guide rolls 12 and 13 and belt 11 passes over guide rolls 14 and 15. The bearings form a hydraulic pressure chamber 18 and 19 above and below the belts. Sealing walls are provided at the edges of the chambers 18 and 19 to prevent excessive escape of liquid, and the wall on the oncoming side of the chamber is shown at- 20 for the top bearingand at 22 for the lower bearing. The wall at the offrunning side of the chamber is shown at 21 for the upper bearing and at 23 for the lower bearing. Suitable seals such as Teflon sliding members or sion occurs. This expansion time is preferably kept at a minimum and will be decreased as the width of the sills 21 and 23 is narrowed.

FIG. 2 shows the time pressure relationship as the web passes through the pressing zone. During the period indicated by the portion of the curve 26, the web is under pressure. As it leaves the pressure zone, pressure drops off as indicated by the expansion portion of the curve 27, with expansion beginning at the location of the broken line 28.

The period of time of compression is necessarily longer than the period of time of expansion, and preferably substantially longer having a ratio of to one or greater.

The permeable belt could be a porous stainless steel belt of .010 inches thickness, for example. Other exam ples would be a low permeability surface covering on the felt or small pore size plastic belts. The permeability can be achieved by porosity of the belt material or by perforation or fabrication of a belt.

If the permeability of the permeable belt is made very high, then its presence has very little influence on the flow conditions in the compression-expansion cycle of wet pressing. Typical moistures'might be as follows:

lngoing moisture 83% Moisture at end of compression cycle 43% Outgoing moisture 61% On the other hand, if the permeability of the belt is made very low, then very little water penetrates the belt during the compression cycle and practically none flows back during the expansion cycle. Typical values might be:

lngoing moisture 83% Moisture at end of compression cycle 80% Outgoing moisture 80% There is indicated, therefore, a permeability of the belt which would achieve a maximum water removal in a wet press by allowing optimum water to be expressed during compression (at the long time interval) while minimizing the rewet water returnedduring the expansion cycle (at the short time interval). A typical set of moisture values for a twenty to one time interval (compression to expansion) are given below:

Permeability, lngoing Minimum Moisture Outgoing cm Moisture Before Expansion Moisture 50X l0 83 4] 83 5 X IO 83 41 53 tional two roll presses with equal time intervals of compression and expansion.

FIG. 3 illustrates another structural arrangement which may be used wherein the extended nip is provided by passing a web through a nip formed between two surfaces provided by a backing roll 30 and a tensioned nonporous belt 32. The belt is provided with tensioning means not shown, and wraps an arc of the roll 30 so that the pressingzone extends from the time of contact of the belt 32 with the web and the roll until it separates from the web on the roll. The paper web is shown at W, and a felt F is passed over the surface of the roll 30. The roll may be provided with grooves. shown schematically at 34 to aid in the flow of water from the web into the felt and the grooves are necessarily made sufficiently small so as'to avoid marking of the paper web W. The permeable barrier sheet S is positioned between the felt and the paper, and as in the structure of FIG. 1, inhibits the flow of water. However, the length of residence time in the nip is sufficiently long to permit adequate flow of water into the felt. When the belt 32 is separating from the roll and the web expands during the release or expansion time, the water flow deterrent or inhibition properties of the sheet prevent water from passing from the felt back to the web.

In the arrangement of FIG. 4, the felt is constructed of a nature so that it can withstand tension and is used for applying the surface pressure to the web. The felt F is provided with tensioning means shown by the arrowed lines 34a to pull up taut over an arc of wrap on the roll 30'. The paper web W is wrapped directly on the smooth outer surface of the roll 30. A permeable barrier sheet S passes between the web and the felt. The barrier sheet has the same properties of porosity as above described in connection with the structures of FIGS. 1 and 3.

In some instances, it would be desirable to provide a felt having a fabric backing capable of exerting tensions that would be in the range useful for pressing water from the paper with the barrier sheet integral with the felt. This arrangement is shown in FIG. 5.wherein the controlled permeable barrier is shown at S as one layer of the felt with the other layer being shown at F. The

web W is laid against the smooth outer surface of the roll 30 and by tensioning the integral felt, pressure is applied to the web over the extent of the pressing nip.

Another form of structure contemplated is illustrated in FIG. 6 wherein a roll 32a is wrapped by belt 34b to perform an extended nip pressing zone with the web W passing through the zone. Between the belt 345 and the web is a felt 33 which is of a critical construction in that it has a flow-resistant" composition. As used herein a flow-resistant shall mean a felt composition having a resistance to water flow, or in other words a low permeability of a degree such that the felt would not be feasible for use with a conventional twin roll press. As will be fully recognized by those versed in the paper making art, a felt for use in a conventional press section of the paper making machine must have a certain degree. of permeability to be receptive of water being squeezed from the web during the short length of time the web and felt pass through the press nip. Felt makers provide felts which have this permeability. Felts which reach a degree of low permeability or substantial resistance to the flow of water are unusable in a conventional press nip. It is these felts which are used in the embodiment of FIG. 6 and are referred. to herein and in the claims as flow resistant felts.

A further structure which is contemplated is where the permeable sheet S, such as shown in FIGS, 1, 3 and 4 is hydrophobic; The felt will be of a hydrophilic nature. The hydrophobic characteristic of the sheet S will resist the flow of water therethrough but over the time afforded in the extended nip the water will be forced through the hydrophobic sheet. On the off-running side, however, the hydrophobic nature of the sheet will prevent the return of water from the felt to the sheet and the hydrophilic nature of the felt will retain the wa- 'ter.

. drophobic material. A preferred form of materialis that which is known under the trademark Silanox, manufactured and sold by Cabot Corporation of Boston, Massachusetts. This coating may be placed on either or both sides of the sheet. It is also contemplated to usea hydrophobic plastic for the sheet. Polytetrafluoroethylene', (Teflon), is a preferred form of plastic. The sheet will be of relatively thin otherwise impermeable material such as Teflon. The small openings will have a diameter on the order of magnitude of the thickness of the sheet; The spacing between the openings will range from two times the sheet thickness to ten times the sheet thickness. As an example, with the use of a sheet having a thickness of .005 inches, the holes will have a diameter of .005 inches with the spacing between the holes ranging frorn .01 inches to .05 inches. Such a sheet would have the effect of introducing flow resistance into the system, and would further have the effect of minimizing the fluid film remaining next to the paper web after it is compressed, limiting-that film approximately to the water which could reside in the small sparsely spaced holes. i

The felt has a hydrophilic characteristic, and in certain circumstances, this should be enhanced by the treatment of the felt with surface active agents or wetting agents. A permeability for wet feltwhich is preferred is that having an air-permeability at l/2 inch water pressure difference less than 10 cubic feet per minute per square foot.

We claim asour invention:

1. The method of pressing liquid from a traveling fibrous web whilethe web passes through an extended press nip which comprises,

, applying pressure to the "traveling web over a compression time for squeezing liquid therefrom and thereafter releasing the pressure with the compression time substantially exceeding the release time,

collecting the liquid squeezed from the web in a liquid receiving means traveling adjacent the web surface, I and thereafter inhibiting the rewetting of the web by a return flow of liquid between the liquid receiving means and the web by placing a porous sheet therebetween having a permeability in the range of 5 X l0 to 0.1 X so thata substantially greater amount of the liquid will flow from the web during the longer compression time than will return to the web during the release time. 2. A press mechanism for removing liquid from a traveling'fibrous web which comprises,

side.

- an extended press nip having opposed pressing surfaces for receiving a traveling web and applying pressing force'to the web'uniformly thereacross with the web being released on the offrunning side of the nip,

the period of time of application of the pressure being larger than the release time,

a traveling felt means passing through the nip with the web for receiving moisture pressed from the web,

and a thin porous sheet passing through the nip between the felt and the web having distributed openings to reduce the rate of flow of liquid from the felt back to the web on the offrunning side, said sheet having a permeability in the range of 5 X 10 to 0.1 X 10 whereby the flow ofliquid from the web to the felt means during the period of time of application of the pressure is substantially greater than the return flow during the time. time 3. A press mechanism for-removing'liq'uid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means is hydrophilic and said porous sheet has a hydrophobic characteristic.

4. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means has been treated with a wettingagent.

5. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is constructed of polytetrafluoroethylene.

6. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is coated with a hydrophobic material.

7. Apress mechanism for removing liquid from a travelingfibrous web constructed in accordance with claim 2 wherein said porous sheet is integral with the felt and forms a layer between the web and the more absorbent outer portion of the felt.

8. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said pressing surfaces are formed by a pair of traveling belts each having a fluid backing pressure chamber for transmitting fluid pressure to the web through the belts.

9. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said pressing surfaces are formed by a supporting rotating roll on one side of the web and a tensioned belt wrapping an arc of the roll on the other 10. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 whereinsaid felt means and said sheet are integral and form one of the surfaces for applying a pressa ing force to the web and the other surface is formed by a rotating roll with the felt wrapping an arc of the roll, and including means for tensioning the felt.

I]. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said sheet is formed of a stainless steel.

12. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said sheet is formed of a plastic material.

13. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein the porous sheet has permeability on 5 20 to one.

the order of .5 X 

1. THE METHOD OF PRESSING LIQUID FROM A TRAVELLING FIBROUS WEB WHILE THE WEB PASSES THROUGH AN EXTENDED PRESS NIP WHICH COMPRISES, APPLYING PRESSURE TO THE TRAVELLING WEB OVER A COMPRESSION TIME FOR SQUEEZING LIQUID THEREFROM AND THEREAFTER RELEASING THE PRESSURE WITH THE COMPRESSION TIME SUBSTANTIALLY EXCEEDINGLY THE RELEASE TIME, COLLECTING THE LIQUID SQUEEZED FROM THE WEB IN A LIQUID RECEIVING MEANS TRAVELLING ADJACENT THE WEB SURFACE, AND THEREAFTER INHIBITING THE REWETTING OF THE WEB BY A RETURN FLOW OF LIQUID BETWEEN THE LIQUID RECEIVING MEANS AND THE WEB BY PLACING A POROUS SHEET THEREBETWEEN.
 2. A press mechanism for removing liquid from a traveling fibrous web which comprises, an extended press nip having opposed pressing surfaces for receiving a traveling web and applying pressing force to the web uniformly thereacross with the web being released on the offrunning side of the nip, the period of time of application of the pressure being larger than the release time, a traveling felt means passing through the nip with the web for receiving moisture pressed from the web, and a thin porous sheet passing through the nip between the felt and the web having distributed openings to reduce the rate of flow of liquid from the felt back to the web on the offrunning side, said sheet having a permeability in the range of 5 X 10 10 to 0.1 X 10 10 whereby the flow of liquid from the web to the felt means during the period of time of application of the pressure is substantially greater than the return flow during the time. time
 3. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means is hydrophilic and said porous sheet has a hydrophobic characteristic.
 4. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means has been treated with a wetting agent.
 5. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is constructed of polytetrafluoroethylene.
 6. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is coated with a hydrophobic material.
 7. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is integral with the felt and forms a layer between the web and the more absorbent outer portion of the felt.
 8. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said pressing surfaces are formed by a pair of traveling belts each having a fluid backing pressure chamber for transmitting fluid pressure to the web through the belts.
 9. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said pressing surfaces are formed by a supporting rotating roll on one side of the web and a tensioned belt wrapping an arc of the roll on the other side.
 10. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means and said sheet are integral and form one of the surfaces for applying a pressing force to the web and the other surface is formed by a rotating roll with the felt wrapping an arc of the roll, and including means for tensioning the felt.
 11. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said sheet is formed of a stainless steel.
 12. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said sheet is formed of a plastic material.
 13. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein the porous sheet has permeability on the order of .5 X 10
 10. 14. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein the ratio of period of time of application of pressure to the release time is on the order of 20 to one. 